The embodiments described herein relate generally to the field of sensing and measuring electronic fields in earth. In this application, the term “earth” is used to refer generally to the lithosphere of the Earth, and more generally to any region wherein a geophysical survey might be conducted. This lithosphere may comprise one or more of any naturally occurring materials such as soil, sand, rock, dry salt, or permafrost, or be man-made, such as asphalt or concrete.
In embodiments of the present invention disclosed herein, the term “electrical conductivity” is used even though electrical resistivity is the inverse of electrical conductivity and the two terms can be interchanged without any loss of meaning or generality. Due to physical-chemical polarization processes that accompany current flow in the earth, the electrical conductivities of earth materials such as rocks and/or fluids are complex and frequency-dependent. One or more embodiments of the present invention disclosed herein can be applied to measuring electric fields for interpretation as the conductivity and/or dielectric permittivity and/or induced polarization relaxation terms such as, but not limited to, chargeability and/or time constant and/or frequency constant, of geological structures and/or man-made objects.
Electromagnetic (EM) soundings probe electrical conductivity as a function of lateral position and depth in the earth. Geological structures and/or man-made objects of interest include, but are not limited to, mineral deposits, hydrocarbon reservoirs, Enhanced-Oil-Recovery/Improved-Oil-Recovery injected fluids and in-situ fluids, hydrofracturing injected fluids and slurries, groundwater reservoirs, fluid fronts, contaminants, permafrost, weathered layers, infrastructure, tunnels, and underground facilities. Since the conductivities of such objects and the surrounding media are generally quite dissimilar, they can, in theory, be discriminated by means of measurement of the subsurface conductivity. Using this methodology, the depth, thickness, and lateral extent of objects of interest can be determined, depending on the availability of naturally occurring EM sources, or controlled-source EM sources such as a transmitter.
Soundings as related to the invention are primarily targeted at objects at a depth of more than 30 m below the earth's surface. This focus on deeper targets requires frequencies that are generally less than 10 kHz, and most often below 100 Hz. In particular, the requirement for depth and low frequency distinguishes the present invention from apparatuses for resistivity mapping that aim to identify and locate features in the first 10 m, and more preferably the first 5 m, of the earth's surface. In these shallow applications, the requirements for measurement sensitivity at low frequencies and reproducibility are low, and some of the improvements necessary for deep soundings that are provided by the invention may not be needed.
A number of measurement scenarios for sounding are employed, including natural and/or controlled electric and/or magnetic sources with many different source and/or receiver combinations and/or geometries for surface-based configurations, borehole-to-surface configurations, surface-to-borehole configurations, single borehole configurations, and multiple borehole (e.g., cross-borehole) configurations. The principal natural source, or passive, sounding methods include the magnetotelluric (MT) methods, such as audio-magnetotelluric (AMT) and magnetovariational (MV) methods, in which the electric and/or magnetic amplitudes of long-period waves from natural EM sources such as lightning discharges and geomagnetic pulsations are monitored near the surface of the earth in order to determine the subsurface electrical impedance as a function of depth. Controlled-source EM methods include both frequency-domain and time-domain measurements of the fields in response to artificially generated EM fields. In time-domain EM surveys routinely practiced by industry, an antenna measures magnetic fields generated from subsurface currents induced in the earth due to an inductive EM source such as a loop. In electrical resistance tomography (ERT) or resistivity surveys routinely practiced by industry, an array of receiver electrodes measures voltage and/or electric fields generated from subsurface currents induced in the earth by an array of transmitter electrodes. In induced polarization (IP) or spectral induced polarization (SIP) surveys routinely practiced by industry, an array of receiver electrodes measures voltage and/or electric fields generated from subsurface currents induced in the earth by an array of transmitter electrodes. In magnetic induced polarization (MIP) or magnetometric resistivity (MMR) or sub-audio magnetic (SAM) surveys routinely practiced by industry, an array of receiver electrodes measures voltage and/or electric fields generated from subsurface currents induced in the earth due to an array of transmitter electrodes. In controlled-source audio-magnetotelluric (CSAMT) or controlled-source magnetotelluric (CSMT) surveys routinely practiced by industry, an array of receiver electrodes measures voltage and/or electric fields and an array of receiver magnetometers measures magnetic fields generated from subsurface currents induced in the earth due to an array of transmitter electrodes.
A common factor in the aforementioned EM methods is the need for a low noise electric field measurement. In the simplest case, the local electric potential is measured in two locations by electrically conducting electrodes buried at or near the earth's surface. The difference between these measurements divided by the separation distance between the electrodes gives the electric field along the line of separation. The system or method requires amplification of the small earth potentials, in addition to filtering, digitization, and subsequent analysis. The principal limitation of the method is the coupling of the conducting electrodes to the electric potential within the earth.
The goal of extant geophysical electrodes is to make a low resistance, low electrical noise contact to the earth. Present electrodes fall into two categories depending on the frequency of operation. Above 1 Hz, solid metal electrodes (stainless steel, phosphor bronze) are generally hammered into, or otherwise buried in, the earth, as described by LeBreque and Daily, Assessment of Measurement Errors for Galvanic-Resistivity Electrodes of Different Composition, 73 GEOPHYSICS. No. 2, at 55-64 (March-April 2008). In addition, water or saline solution is often added to the earth to reduce the contact resistance to the electrode and the electrical resistance of the earth in the immediate vicinity of the electrode. Below 1 Hz, metal/metal salt combination electrodes, such as silver/silver chloride (Ag/AgCl), copper/copper sulfate (Cu/CuSO4), or lead/lead chloride (Pb/PbCl2), are buried in excavated holes. The metal electrode is encased in a pot filled with wet mud (e.g. bentonite) that contains the required ions, such as silver (Ag), copper (Cu), lead (Pb), and chlorine (Cl). For improved performance, the pot is buried in a hole, backfilled by the original earth material or specialized earth material substitutes, mixed with electrolyte. The porous pot couples to the prepared earth by means of the salt solution slowly leaking into the surrounding environment through a porous section of the pot.
FIG. 1 depicts a cross sectional view of a conventional metal/metal salt electrode. For convenience, both solid metal and metal/metal salt electrodes are termed “electrochemical” herein because they rely on an exchange of ions with the earth in order to transfer electric charge, and thereby measure the local electric potential. A further common factor is that solid metal and metal/metal salt electrodes are prepared with the goal of having an electrical contact resistance to the earth of less than 1 kΩ. The electrode of FIG. 1 comprises a body 125 capped with a top cap 100, with solder insulated with heat shrink tubing 110 and hot melt glue 115 holding in place a lead wire 120. Clay mud with electrolyte 130 fills the body 125. The lower end of the body is fitted with a porous plug 140, with a partial PVC block forming a channel 135 above the porous plug 140.
A detailed discussion of the chemical processes and design issues associated with electrochemical electrodes is given by Petiau, Second Generation of Lead-Lead Chloride Electrodes for Geophysical Applications, 157 PURE AND APPLIED GEOPHYSICS, at 357-382 (2000) (“Petiau”). The fundamental issue is that the main part of the potential of an electrochemical electrode is given by the Nernst formula:
                    V        =                              V            o                    +                                    RT              nF                        ⁢            ln            ⁢                                                  ⁢                          a                              M                +                n                                                                        [        1        ]            where Vo is the standard potential at 25° C., R is the gas constant, T the temperature, F the Faraday constant (RT/F=25.7 mV at 25° C.), n the metal valency, and aM+n is the metal activity which can be represented by the concentration of the metal ion in solution. The various terms in Equation 1 lead to many practical issues. These issues are most pronounced for metal/metal salt electrodes but also apply to solid metal electrodes, and include the following:                a. Inability to work in very dry earth. An electrochemical reaction requires the migration and transfer of ions to and from the electrode surface, which requires some level of moisture. A porous pot provides a medium and also stabilizes the interface electrical potential at the electrode. However coupling the pot to the surrounding soil is still an issue, particularly in areas of dry, well drained sand, gravel, and/or caliche, because fluid in the pot depletes quickly and inconsistently, leading to failure. Similarly, adding water to the earth only lasts for a limited period of time and results in an electrical connection to the earth that is continually changing.        b. Limited operational lifetime due to electrode degradation. Metal electrodes exposed to earth generally corrode. For porous pots there is a gradual reduction in the salt concentration with the rate of leakage affected by earth conditions and even air humidity. Solution leakage can be addressed by refilling and reburying the electrodes, which otherwise can be used for only 3 to 4 days. Recently electrodes with the salt dissolved in a hard gel have been introduced. (See, for example, Petiau, at 359.) These electrodes, however, must still be buried in mud made with salt water, which will also dissipate over time.        c. Susceptibility to local earth chemistry and conditions. Dissolved ions can affect the interface potentials between dissimilar materials, and damp earth water flowing into the electrode can dilute the salt solution and thereby directly affect the contact resistance and the DC potential (see Equation 1).        d. Temperature drift. This is an inherent problem in electrodes that couple via an electrochemical reaction (see Equation 1).        e. Uncertainty in the proportion of the earth potential that is recorded by the overall system. The input impedance of the first stage amplifier of the data acquisition system and the electrical impedance between the electrode and the earth form an impedance divider network. Variations in the electrode-to-earth impedance cause variations in the amplitude of the recorded signal.        f. Increased noise. Electrochemical reactions have an associated noise, also termed corrosion noise. This has been identified as a limiting factor in underwater geophysics, and is seen in many types of solid metal and metal/metal salt electrodes.        g. Increased setup time. This is due to the need for burial in prepared earth to provide an adequate surface area for a low impedance contact and the time required for the (electrode+earth) system to equilibrate.        h. Environmental and permitting issues. Installation of metal salt electrodes often requires obtaining necessary regulatory approvals/permits, and some electrodes (e.g. cadmium/cadmium chloride (Cd/CdCl2), lead/lead chloride (Pb/PbCl2)) are not allowed in some locations for reasons of environmental contamination. Also, installation of electrodes requires environmental disturbance and invasion. In some environmentally sensitive locations, even digging a hole large enough for a small metal rod electrode requires a lengthy permitting process and complete restoration of the earth after the survey.        
The issues listed above are well known and in part set the limits of present EM methods in geophysics. For applications related to long duration monitoring, these issues become serious practical problems that must be addressed. Specifically, it is desirable that the electrodes be emplaced for periods on the order of months to years, depending on the application, compared to the days and weeks standard today, and the variation in the recorded signal due to the properties of the measurement system must be low enough to see very slow, small changes in earth potential over time. Further, it is desirable that the electrodes be robust to changes in earth water content in order to be used as an operational monitoring tool, and the measurement noise must be low enough to collect information a depths of order 10 km.
Other practical issues arise because the earth potentials of interest in electromagnetic soundings are small, on the order of 1 μV, and are typically measured by electrodes spaced 20 m to 100 m apart, necessitating the transmission of very small voltages over a significant distance. The wires used to transfer these voltages to the data acquisition system are susceptible to being buffeted by the wind, which causes the wires to move in the earth's magnetic field and thereby induces spurious voltages to the measurement. The wires can be pinned to the earth to minimize wire motion, but such installation adds time and cost to the survey. Secondly, charged dust particles blown by the wind can induce image charges in the wire that also result in spurious voltages.
Further, the wire used to carry the earth potential signal can act like an antenna, directly picking up electromagnetic interference (EMI). The amount of EMI coupled into the wire depends on the input impedance of the first stage of the data acquisition system. Ideally this input impedance is high in order to provide a degree of immunity to changes in the electrical resistance between the electrode and the earth. However, the higher the input impedance, the higher the EMI that is picked up, resulting in a trade-off. Thus, even when a reliable coupling to the earth has been achieved with an electrochemical electrode, the overall measurement is prone to many additional practical problems.
Based on the above, there exists a need for an electric field measurement system for geophysical soundings that can couple to the earth without involving an electrochemical reaction or needing a low resistance electrical contact. Applications of significant commercial and research interest exist in areas too dry or too cold to be surveyed by existing methods. In addition, an electric field measurement system that is not limited by the physical effects associated with electrochemical coupling would enable new long-term applications including but not limited to the monitoring of: the geological integrity of reservoirs used for CO2 sequestration, hydrocarbon production from reservoirs including gas and water injection, hydrofracture injection of fluids and slurries, fresh water storage, mineral production from in-situ mining, acid mine drainage, contaminants, groundwater, and/or infrastructure integrity. Accordingly, a need is present for methods, systems and apparatuses to sense and measure earth potential using sensors with components that can couple to the earth without having electrochemical reaction with the earth and/or overcome issues discussed above.